Tetrodotoxin Toxicity

Etiology

The toxin is found naturally in various vertebrates and invertebrates with no close phylogenetic relationship. Of terrestrial vertebrates, the toxin is found in Western, rough-skinned newts of the genus Taricha, the Eastern Newt (Notophthalmus viridescens), and toads of the genus Atelopus. In marine vertebrates, the toxin is found in over 20 pufferfish species[1] and certain angelfish. Mollusks containing the toxin include several species of the blue-ringed octopus, Niotha gastropods, and the genus Naticidae (moon snails). Other invertebrates that contain the toxin include several starfish species, several species of xanthid crabs, species of the phylum Chaetognatha (arrow worms), species of the phylum Nemertea (ribbon worms), some flatworms, and planarians of the genus Bipalium.[6][7][8]

Many distantly related taxa have evolved the ability to produce tetrodotoxin because the animals themselves do not produce the toxin. Bacteria produce a toxin that accumulates in animals higher up the food chain. This was discovered in an experiment by collecting pufferfish raised in captivity and measuring levels of tetrodotoxin in their organs, particularly the liver. All captive specimens had undetectable levels of tetrodotoxin. Some of these captive pufferfish were fed the livers of wild pufferfish, leading to accumulated tetrodotoxin. 

The bacteria implicated in the production of tetrodotoxin include members of the following genera: Pseudoalteromonas, Pseudomonas, Vibrio, Aeromonas, Alteromonas, Shewanella, Roseobacter, Raoultella, Actinomycetes, Microbacterium, and Serratia.[9][10] Some animals have tetrodotoxin-producing bacteria in their natural microbiome, contributing to the toxin accumulation. The exception is newts, and it is unclear whether the tetrodotoxin is produced endogenously or exogenously due to conflicting results from experiments.

Animals that contain tetrodotoxin are resistant to the neurological effects of the toxin. Typically, sodium channels have an aromatic amino acid chain in the P-loop region of domain I. Animals that accumulate tetrodotoxin in their bodies have a non-aromatic amino acid substitution that causes the sodium channel to have a low affinity for tetrodotoxin. Therefore, sodium channels in these species are immune to tetrodotoxin.[11] Garter snakes, which do not contain tetrodotoxin but prey on toxic newts that do, have also acquired this mutation.[12] Additionally, tetrodotoxin-binding proteins that filter out the toxin exist in some animals, such as the shore crab, pufferfish, and gastropods.[8]

Poisoning in humans occurs when they ingest tetrodotoxin-containing organisms. In the United States, the most common sources of poisoning are pufferfish imported from Japan and Mexico, pufferfish mislabeled as another fish, Pacific newts, and the Eastern newt. There are 4 Pacific newts of the genus Taricha species that contain the toxin. They are distributed along the Pacific coast from Southern Alaska to Baja, Mexico. The most toxic newts are found in Oregon. A single species of newt on the east coast, the Eastern newt (Notophthalmus viridescens), contains the toxin.[13] The toxin produced by the Eastern newt is only 1/100th as potent as that found in Pacific newts.

Worldwide, the sources of toxicity vary based on local wildlife. Pufferfish are implicated in the poisoning of patients in Bangladesh, Japan, Australia, and India, in addition to the United States. Toadfish are another source of poisoning in Australia. Ingestions of marine gastropods and thread-sail fish are additional sources of toxicity in Japan. Poisonings from marine gastropods have been reported in China, Japan, and Taiwan. People who have eaten the eggs of horseshoe crabs have been poisoned in Thailand. Multiple dogs in New Zealand were poisoned after eating grey side-grilled sea slugs.[14]

Epidemiology

The incidence of tetrodotoxin poisoning is rare and variable depending on the geographic locale and organism involved. The most commonly implicated exposures affected pufferfish, and the incidence is higher in countries where people eat pufferfish regularly, such as Japan, Taiwan, and some Southeast Asian countries. From 2002 to 2006, 223 Japanese patients suffered tetrodotoxin poisoning, and 13 of these patients died. From 2001 to 2006, 53 patients in Singapore were diagnosed with tetrodotoxin poisoning, and 8 of these patients died.[15] In 2008, cheap pufferfish sold in fish markets led to 3 outbreaks in Bangladesh, affecting 141 people, 17 of whom died from respiratory arrest. An additional outbreak resulted from the ingestion of the roe of Takifugu oblongus, resulting in 5 deaths.[16] Another cluster of patients was reported in Oman in 2018, where 5 patients suffered poisoning after frying locally caught pufferfish.[17] Poisonings involving other tetrodotoxin-containing species are rare, as these animals are generally not edible. Case reports document exposure to poisonous species of newts, often kept as exotic pets,[18][19] and to xanthid crab species.[20]

Pathophysiology

At room temperature, tetrodotoxin is a colorless, crystalline substance with weak basic properties. Tetrodotoxin functions by inhibiting voltage-gated sodium channels. This occurs by interacting with the positively charged guanidine group on tetrodotoxin and the negatively charged carboxylate groups in the mouth of the sodium channel pore.[11] The blockade effect does not change the resting potential of the neuronal membrane but rather prevents the influx of sodium through the channel, effectively preventing transmission of an action potential. The blockade disrupts the function of the brainstem and motor, sensory, and autonomic nerves. This subsequently leads to gastrointestinal, cardiac, and neurologic dysfunction.

Toxicokinetics

Tetrodotoxin is rapidly absorbed from the gastrointestinal tract.[21] The distribution following ingestion is not well described in the literature. One study looking at the distribution of subcutaneous tetrodotoxin found a peak concentration of 1.5 hours and half-life of 4.5 hours at all doses studied, with the maximum dose studied being 45 μg.[22] 

History and Physical

The patient’s history will involve the consumption of tetrodotoxin-containing organisms, such as pufferfish, newts, or sea snails. The onset and severity of symptoms in tetrodotoxin poisoning depend on the time since ingestion and the dose administered. In most cases, symptoms usually occur within 30 minutes of ingestion. A few cases report symptoms starting after 20 hours.[23][24][25][26]

Mild toxicity can have symptom onset between 30 minutes to 6 hours and general recovery after 24 hours. More severe toxicity can lead to rapid onset of symptoms and progression to respiratory failure within 15 to 20 minutes.[27]

There are 4 grades of poisoning based on a scale created by Fukuda and Tani in 1941:

• Grade 1: Paresthesias and perioral numbness, with or without gastrointestinal symptoms (nausea, vomiting, abdominal pain, and diarrhea)
• Grade 2: Facial numbness, slurred speech, early motor paralysis, and incoordination, but with normal reflexes
• Grade 3: Generalized flaccid paralysis, aphonia, respiratory failure, and fixed or dilated pupils (in a conscious patient)
• Grade 4: Severe respiratory failure with hypoxia, bradycardia, hypotension, cardiac dysrhythmias, and unconsciousness

Evaluation

Testing should be directed at evaluating other potential etiologies for a patient's presenting symptoms. These may include an electrocardiogram to assess chest pain and tachycardia, a computed tomography (CT) scan of the brain to evaluate for potential stroke or intracranial pathology, and bloodwork to evaluate electrolyte status, kidney function, and blood counts.

No specific laboratory tests exist to confirm tetrodotoxin poisoning in the acute clinical setting. Therefore, diagnosis is largely based on history and symptoms. In the research laboratory setting, however, mouse bioassays and liquid chromatography coupled with mass spectrometry can be used to detect tetrodotoxin. Confirmatory testing by LC-MS/MS can be performed by sending urine or serum testing to specialized labs, such as at the CDC, to measure levels of tetrodotoxin.[17][28] Decisions about clinical care should not be delayed waiting for these confirmatory tests.

Treatment / Management

No known antidote exists for tetrodotoxin poisoning. The mainstay of treatment is respiratory support and supportive care until the tetrodotoxin is eliminated.

If ingestion occurs within 60 minutes and no contraindications, such as decreased mental status or active vomiting, are present, activated charcoal and/or gastric lavage may be administered.

Hemodialysis may be helpful in patients with renal disease, but the evidence supporting toxin removal is conflicting. Tetrodotoxin is not water-soluble enough to benefit from hemodialysis.[29] A case report from Japan describes a patient with renal dysfunction and uremia who recovered after receiving multiple sessions of hemodialysis despite persistent mild toxicity symptoms.[30] Another case series of 5 patients supports toxin removal by hemodialysis.[17] Another case series found that hemodialysis may have improved recovery, but the information provided was inadequate to support this intervention.[31](B3)

A monoclonal antibody against tetrodotoxin (anti-tetrodotoxin) exists [32] and has neutralized tetrodotoxin in mice.[33] No reports regarding humans are available.(B3)

Neostigmine has been used to treat acute respiratory failure from tetrodotoxin poisoning.[34] However, a recent review found insufficient data to provide evidence for or against this practice.[35](B3)

For suspected tetrodotoxin poisoning, patients should be observed in the intensive care unit due to the possibility of delayed symptom onset, which can occur up to 20 hours postexposure.

Differential Diagnosis

The list of differential diagnoses includes the following:

• Ciguatera toxicity
• First-degree AV block
• Guillain-Barré syndrome
• Hypocalcemia
• Lambert-Eaton myasthenia syndrome
• Octopus envenomation
• Second-degree AV block
• Shellfish toxicity
• Third-degree AV block

Prognosis

Left untreated, patients who have severe toxicity and develop cardiac complications or respiratory paralysis will die. With sufficient supportive care, most patients fully recover without permanent neurologic sequelae. 

Complications

Tetrodotoxin toxicity is commonly associated with the ingestion of pufferfish or certain other marine organisms, and can lead to severe and potentially fatal complications. Complications associated with tetrodotoxin toxicity: [17]

Neurological Symptoms

Tetrodotoxin primarily affects the nervous system. Symptoms may include numbness and tingling, especially around the mouth and extremities. Paralysis, starting with the face and spreading to other parts of the body, can occur. Muscle weakness and difficulty speaking or swallowing may also be observed.

Respiratory Distress

Tetrodotoxin can lead to respiratory muscle paralysis, causing difficulty in breathing. Severe cases may result in respiratory failure, requiring immediate medical intervention, such as mechanical ventilation.

Cardiovascular Effects

Tetrodotoxin can cause a drop in blood pressure, leading to hypotension. Bradycardia (slow heart rate) or arrhythmias may occur, contributing to cardiovascular instability.

Gastrointestinal Symptoms

Nausea, vomiting, and abdominal pain may manifest shortly after ingestion of tetrodotoxin-containing seafood.

Seizures

In some cases, tetrodotoxin toxicity can lead to seizures, adding to the neurological complications.

Coma

Severe poisoning can result in a state of unconsciousness and coma.

Death

Tetrodotoxin toxicity can be fatal, primarily due to respiratory failure or cardiovascular collapse.

Delayed Onset

Symptoms may take several hours to appear after ingestion of contaminated seafood, which can complicate diagnosis and treatment.

No Antidote

No specific antidote for tetrodotoxin poisoning exists. Treatment mainly involves supportive care and symptom management.